In recent years, for example, a wireless power transmission system that supplies electric power to a portable apparatus such as a smart phone and a laptop personal computer wirelessly has been put to practical use. As the wireless power transmission system, there is a wireless power transmission system disclosed in Patent Document 1, for example.
Patent Document 1 discloses an electromagnetic induction-type wireless power transmission system. The electromagnetic induction-type wireless power transmission system includes a power transmission device and a power receiving device. The power transmission device includes a power transmission coil, the power receiving device includes a power receiving coil, and electric power is transmitted between these coils by electromagnetic induction.
Patent Document 2 discloses an electric field coupling-type wireless power transmission system. The electric field coupling-type wireless power transmission system includes a power transmission device and a power receiving device. The power transmission device includes a power transmission electrode, the power receiving device includes a power receiving electrode, and electric power is transmitted between these electrodes by electrostatic induction.
In the above-mentioned systems, electric power that is transmitted from the power transmission device is required to be controlled to a value appropriate for the power receiving device in consideration of an operation voltage, a withstand voltage, and the like of the power receiving device. In the power transmission system as disclosed in Patent Document 1, in order to control the electric power that is transmitted to the value appropriate for the power receiving device, wireless communication circuits are provided on the power transmission device and the power receiving device and transmit and receive a control signal to and from each other.
Patent Document 1: Japanese Patent No. 3344593.
Patent Document 2: International Publication Pamphlet No. WO2011/148803.
In the power transmission system as disclosed in Patent Document 1, in order to control the electric power that is transmitted to the value appropriate for a state of the power receiving device, the communication circuits for making communication with each other are required to be provided on the power transmission device and the power receiving device. For this reason, the configurations of the power transmission device and the power receiving device become complicated.